Van Gaal was taking charge of his first competitive game as United manager, but his new side exhibited many of the same failings from last season — when they finished seventh — as they slumped to a surprise defeat.

Man U skipper Rooney (No 10) scores to make it 1-1 (53rd minute)

South Korea midfielder Ki Sung-yueng exploited inattentive marking to give Swansea a 28th-minute lead with a left-foot shot from the edge of the box. United’s new captain Wayne Rooney equalised early in the second period, but Gylfi Sigurdsson condemned the hosts to a first opening-day home defeat since 1972 with a close-range finish in the 72nd minute.

Swansea's Gylfi Sigurdsson (in white) shoots the winner (72nd minute) against Manchester United at Old Trafford on Saturday. Pic/Getty Images.

“It is very disappointing that we have lost our first home match. “We were very nervous in the first half, made the wrong choices, and that is a pity. In the second half, we never played as a team. Therefore, I’m responsible,” said Van Gaal, who took over at Utd after leading Holland to the World Cup semis.

United bossed possession in the first half, but Swansea struck from their first sight of goal when Ki stroked home a lay-off from Sigurdsson. Van Gaal switched formations from a 3-4-1-2 to a 4-2-3-1 at half-time and United responded immediately, with Rooney acrobatically volleying in at the back post after Juan Mata’s corner was flicked on by Phil Jones. Rooney then hit the post with a free-kick, but Swansea claimed victory when Icelandic midfielder Sigurdsson — back at the club after two years at Tottenham — slammed home Wayne Routledge’s mishit volley.